Pope Francis has a respiratory infection and will need to spend a few days in hospital in Rome, the Vatican says.
The 86-year-old had breathing difficulties in recent days but does not have Covid, a statement said.
It said he would need “a few days of appropriate hospital medical therapy”.
“Pope Francis is touched by the many messages received and expresses his gratitude for the closeness and prayer,” the statement added.
His closest staff, including security, are expected to stay the night at the Gemelli Hospital, a person with direct knowledge told the BBC.
This is the busiest time of the year for Pope Francis, with many events and services scheduled ahead of Easter weekend.
A Palm Sunday Mass is scheduled this weekend, and Holy Week and Easter celebrations next week.
He is also scheduled to visit Hungary at the end of April.
On Wednesday morning, he presided over his weekly general audience in St Peter’s Square. He appeared in good spirits but was seen grimacing as he was helped into his vehicle.
The Vatican initially said the Pope had gone to hospital for a previously scheduled check-up, but Italian media have questioned this account after a television interview was cancelled at short notice.
Reacting to the news, President Joe Biden asked people to make an “extra prayer” for the Pope’s recovery.